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Teacher Background

Many students (and adults) mistakenly believe the seasons are due to the changing distance between Earth and the sun as our planet progresses in its orbit. According to this (incorrect) scenario, Earth experiences summer when it is closer to the sun and winter when it is farther away. Earth’s distance from the sun does vary over the course of a year. At its near point (perihelion), Earth is 147.9 million kilometers from the sun; at its far point (aphelion), it is 152.09 million kilometers away. However, the difference between the perihelion and aphelion is small—less than 3%.

In fact, the varying distance between Earth and the sun has little impact on our seasons. Perihelion (when Earth is closest to the sun) occurs in early January, when it is winter in the northern hemisphere. Aphelion (when Earth is furthest away) occurs in early July, during summer in the northern hemisphere. But remember, summer in the northern hemisphere occurs when it is winter in the southern hemisphere, and vice versa. Since Earth experiences winter and summer simultaneously, the seasons must not be determined by Earth’s distance to the sun.

The seasons are caused by Earth’s rotation around an axis, an imaginary line through the center of Earth that connects the North Pole to the South Pole. This axis is tilted about 23.5° from a vertical position, relative to its orbit around the sun. During summer in the northern hemisphere, Earth’s North Pole leans toward the sun, while the South Pole leans away. When it is winter in the northern hemisphere, the South Pole leans toward the sun and the North Pole leans away. In between winter and summer, when Earth experiences spring and fall, neither Pole leans toward the sun.

The tilt of Earth’s axis alters the hours of daylight and the apparent angle of the sun in the sky. During summer, there are more of hours of daylight and the sun is higher in the sky than during winter. Thus, summer brings more heating, longer days, and more intense light. Conversely, winter is characterized by fewer hours of daylight and a lower angle of the sun, which combine to produce cooler, shorter days.

The hours of nighttime also affect the seasons. In winter, days are shorter than nights, so there is more time for Earth’s surface to radiate heat back into space. This causes a net decrease in heat in locations experiencing winter. But in summer, days are longer than nights, so there is a net increase in heat in Earth’s surface.

In the northern hemisphere, the longest day of summer, known as the summer solstice, occurs around June 21; the shortest day, or winter solstice,
occurs on or around December 21. Between summer and winter, during the seasons of spring and fall, neither of Earth’s poles leans toward the sun. Days and nights are exactly 12 hours long on both the first day of spring (spring equinox) and the first day of fall (autumn equinox).

Objectives and Standards

Earth and Space Science

• The sun, an average star, is the central and largest body in the solar system.

• Most objects in the solar system are in regular and predictable motion.

• Seasons result from variations in the amount of the sun’s energy hitting the Earth’s surface, due to the tilt of the Earth’s rotation on its axis and the length of day.

Physical Science

• The motion of an object can be described by its position, direction of motion and speed. That motion can be represented and measured on a graph.

• The sun is a major source of energy for changes on the Earth’s surface.

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Science, Health and Math Skills

• Data collection

• Measuring

• Observing

• Drawing conclusions

Materials and Setup

Teacher Materials

• Earth globe (with axial tilt) on a portable stand

• Yellow ball, about 8 in. diameter, to serve as a sun model

• 2 small, blue glass “pony” beads (available from craft stores)

• 2 toothpicks or wooden skewers

• Twine or string, approximately 75 ft

• Glue

Materials per Student or Group of Students

• 8-in. square of cardboard

• 8-in. length of string

• Clear plastic dome-shaped lid (as used to cover whipped toppings on coffee or frozen drinks)

• Fine-point marker, black

• Pencil

• Protractor

• Ruler

• Several pieces of masking tape

• Copies of student sheets

Setup

1. For Part 1, prepare two small Earth models ahead of time, each comprised of a single blue “pony” bead glued to the end of a toothpick or wooden skewer. To conduct the activity, you will need an open space about 75 feet long (e.g., a long hallway, football field, etc.).

2. For Part 3, select two dates on which to conduct observations. Students will need to be outside, in an area that provides a wide, unobstructed view of the sky. Be aware that the activity will not work on a dark or cloudy day because shadows will not be clearly visible.

3. Students will need to align their sun tracking boards on the same point of the same east-west line for all measurements. Seek out a permanent east-west feature (e.g., painted line on a playground, edge of sidewalk, south-facing window ledge, etc.). If no such line is available, use a magnetic compass to sight an east-west line on a permanent concrete or asphalt surface. Mark the line with chalk.

4. On each appointed day, students will take two or three measurements in the morning and two or three more in the afternoon. Make sure students understand how to mark the position of the sun on the plastic dome.

5. Have students work individually or in groups of two.

Procedure and Extensions

Part 1: Earth and Sun Models

Time: 45 minutes

1. Take your students into an open area (long hallway, open outdoor area, etc.), and ask one student to hold the 8-inch yellow ball. This represents the sun. Then, ask a second student to hold an Earth model.

2. Tell your students the two models are to scale (the sun’s diameter is 110 times that of Earth). Ask, To show proper scale, how far apart should the sun and Earth models be from each other? Encourage students to discuss their ideas.

3. Wrap the string around the sun model 33.5 time. Then hold one end of the string next to the sun and have the “Earth” student take the other end of the string and walk away from the “sun” student until the string is fully extended. At that point, you have created an accurate scale model—size and distance—of Earth and the sun.

4. Give another student the second Earth model. Have him or her walk to the first “Earth” student, and then step about 60 centimeters (2 feet) further away from the sun.

5. Have students observe the positioning of the three models. Explain that the two blue beads represent Earth, the yellow ball represents the sun, and the positioning of the models represents the nearest and furthest distances between Earth and the sun. Explain perihelion and aphelion to your students. Mention that the bead closer to the sun represents Earth at the near point (perihelion) in its orbit, which occurs during the northern hemisphere winter/southern hemisphere summer. The more distant bead represents Earth at its far point (aphelion), which occurs during the northern hemisphere summer/southern hemisphere winter.

6. Ask, Does the distance between Earth and the sun cause the seasons? You may help your students to visualize the question by giving the “perihelion Earth” student a small sign that reads “Northern Hemisphere Winter” and the “aphelion Earth” student a sign that reads “Northern Hemisphere Summer.”

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Part 2: Tilted Earth

Time: 45 minutes

1. Return to the classroom with your students. Place the yellow ball representing the sun on a stool in the center of the classroom. Ask for a volunteer to hold the Earth globe that shows the tilt of the axis. Point out that the distances and sizes of the sun model and Earth globe will not be to scale in this part of the activity.

2. Have the student holding the globe walk in a circular counterclockwise pattern around the sun. This pattern represents Earth’s orbit. At all times during the “orbit,” the student should ensure that the globe’s North Pole axis is pointing in the same direction (i.e., toward the same side of the room). Also, have the student gently spin the globe while he or she “orbits,” to remind the class that Earth rotates while it circles the sun.

3. Over the course of several complete orbits, have all students observe how Earth’s axis always leans in the same direction. At one point during Earth’s orbit, the northern end of the axis will lean toward the sun, and the southern end will lean away. Stop the student who is carrying the globe and make sure everyone notes the position of the northern axis at this time. Then, have the “orbiting” student continue. Stop him or her again when he or she reaches the opposite side of the sun. Students now will observe that the north axis leans away from—while the southern axis leans toward—the sun. In between these two positions, the northern and southern axes lean neither toward nor away from the sun.

4. Review the seasons with your class as the student and Earth “globe” orbit the sun.

5. Discuss as a class how Earth’s tilt affects the seasons. Give students a list of cities around the world (e.g., Minneapolis, Sydney, Beijing, Buenos Aires, and Moscow). Have students predict whether residents of each city experience winter or summer in July. Then, have students use the globe to locate each city and verify their predictions.

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Part 3a: Build a Sun Tracking Board

TIme: 60 minutes

1. Provide each student or team of two students with materials and copies of the “Sun Tracking Board” student sheet.

2. Have students follow the instructions to build their sun tracking boards.

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Part 3b: Tracking the Sun

Time: Two days scheduled two to three months apart. For each day, four or more 10-minute sessions will be conducted at 15-minute intervals.

1. Provide each student or team of two students with a copy of the “Sun Tracking Data” sheet. Before noon, take students outside to the preselected location. Have students place their boards on the ground, with the “south” edge squarely on the east-west line identified previously.

2. Have students follow the instructions on the student sheet. They should record the location of the sun’s shadow on the dome of the sun tracking board and record the time of their measurement. Have students use a protractor to estimate the angle of the sun above the horizon, and record the value.

3. Take students outside three or more times that same day (at least twice before noon and twice after noon), and have them make new measurements.

4. After all measurements have been recorded on the dome, have students draw a curved line connecting the dots on the dome and extend the line to the east and west horizons. (It is best to demonstrate this step before the students try it.) Put the boards away in a place where they will not be disturbed.

5. Repeat the steps above two to three months later. This time, students will record a new path on the domes of their tracking boards.

6. After the second set of observations, have students analyze the two paths on their domes by answering the questions at the bottom of the “Sun Tracking Data” sheet.

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Extension

Challenge students to provide possible reasons why portions of the Arctic are known as the “Land of the Midnight Sun.” Help them understand that, during summer in the northern hemisphere, the North Pole and Arctic regions face the sun. At this time of year, areas above the Arctic Circle experience continuous light throughout the day and night (including midnight). Have students hypothesize what happens when the South Pole and Antarctic regions are tilted toward the sun, and what happens in these regions during winter months. Ask students what it would be like to live in continuous light or dark for several months.

Handouts and Downloads

Download color images from The Science of Sleep and Daily Rhythms Teacher's Guide.